Membrane-based exhaust gas scrubbing method and system

ABSTRACT

Various exemplary embodiments relate to a method and apparatus to reduce emissions of target emission gasses such as sulfur oxides, nitrogen oxides, and carbon oxides from combustion exhaust such as marine engine exhaust by gas membrane separation and liquid carrier chemical absorption. The membrane separation system consists of an absorption system containing semi-permeable hollow fiber membranes through which is circulated a liquid absorbent. Exhaust gases contact the exterior surface of the membranes and the target gasses selectively permeate the membrane wall and are absorbed by the liquid carrier(s) within the bore and thereby are removed from the exhaust stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation co-pending PCT application No.PCT/CA2014/050359 filed on Apr. 8, 2014, which claims priority to U.S.Provisional Application No. 61/835,288, filed on Jun. 14, 2013, theentire disclosures of which are incorporated herein by reference.

TECHNICAL FIELD

The invention relates to processing of combustion gasses to removecontaminants such as oxides of sulfur, nitrogen and carbon. Theinvention has particular application to treating exhaust from combustionengines such as marine diesel engines.

BACKGROUND

Marine diesel engines power the majority of ships used for marinetransportation. These engines typically burn Heavy Fuel Oil (HFO), whichcontains high concentrations of sulfur and other impurities. Thecombustion process produces high concentrations of sulfur oxides (SOX),nitrogen oxides (NOX), carbon oxides (COX) such as C0₂, and other gasesthat are subject to increasing restrictions with new emerging emissionsrequirements.

One approach to reducing marine engine emissions is to switch to higherpurified fuels, or distillates. These distillates are more expensivethan HFO. An alternative is to post -treat, clean, or scrub thecombustion exhaust gasses before they are discharged into theatmosphere.

Sea water scrubbers have been developed as a post-treatment solution toclean marine engine exhaust. A commonly used process is to spray aqueousalkaline or ammonia sorbents into the exhaust stream. However, these‘wet’ sea water scrubbers can require large amounts of water andconsequently generate large amounts of waste water, which can includemetal salts such as calcium sulfate, soot, oils, and heavy metals. Thiscan produce a toxic sludge that requires complex on board watertreatment, and as well as disposal of sludge at designated ports. Theresultant system is large, complex, expensive and energy intensive,increasing ship fuel consumption by as much as three percent. Althoughconventional sea water scrubber systems may be well suited for fixedland based power plants, they are simply too large and complex tooperate efficiently in a marine application. As well, such systems maynot be well suited to removing C0₂ from marine engine exhaust.

Treatment of marine exhaust could in principle be accomplished bymodifying existing land-based technology to bubble marine exhaust gasesthrough an ionic liquid. However, this approach may not be practical dueto the high flow rates of marine exhaust and the resultant large volumeof ionic liquid required, in light of the space and weight constraintsof a marine vessel. The energy required to compress the exhaust gases tobubble through the ionic liquids could exceed the total energy availablefrom the ship.

A system for scrubbing marine engine exhaust gasses using membranetechnology has been proposed in Chinese patent No 200710012371.1.

An object of the present invention is to provide an improved method andsystem for reducing the concentration of one or more target emissiongasses from a source such as a marine diesel engine.

SUMMARY

An alternative to the use of a conventional seawater scrubber forremoving unwanted compounds from marine engine exhaust gas is to usemembrane technology to separate and process one or more Target EmissionGasses (TEG's) such as SOX, NOX and/or COX from the exhaust gas.Advantages to using membranes over traditional solvent-based extractionprocesses such as sea water-based scrubbers include being potentiallysmaller, more energy efficient and producing less waste water than aconventional water-based scrubber. Although membrane-based systems havebeen proposed in the past, the present invention relates to improvementsthat render such systems highly effective in a variety of applicationsincluding use with marine vessels.

According to one aspect, the invention relates to a method for reducingthe concentration of a target emission gas (TEG) from a source of engineexhaust gas comprising the steps of:

directing said gas into an enclosed space containing at least one arrayof hollow fibre ceramic membranes, wherein said exhaust gas contacts anexterior surface of said membranes whereupon TEG within said exhaust gasselectively permeates through said membrane thereby lowering theconcentration of said TEG within said exhaust gas;

circulating a carrier liquid capable of retaining said TEG through boresof said hollow fibre ceramic membranes thereby elevating theconcentration of said TEG compounds within said carrier liquid;

discharging said exhaust gas containing a reduced TEG concentration fromthe enclosed space and discharging said liquid from said hollow fibreceramic membrane array, wherein said discharged liquid containsmolecules of TEG dissolved therein.

The liquid can discharged from the membrane assembly into theenvironment in one of an “open” mode of operation or alternatively aclosed loop mode can be used, such as wherein said TEG is separated fromsaid liquid and said liquid is recycled through said membrane array.

The carrier liquid may comprise one of an ionic liquid, sodiumhydroxide, fresh water or seawater. The ionic liquid may comprise atask-specific ionic liquid (TSIL) which is specific to said TEG's. Ifthe carrier liquid is an ionic liquid, the method may comprise thefurther step performed after said liquid enters the discharge conduit,of separating said TEG from said carrier liquid for storage andrecycling said carrier liquid through said membranes.

The TEG may comprise one or more of a sulfur oxide, a nitrous oxide or acarbon oxide such as CO2.

The method may include the further steps of determining theconcentration of TEG within untreated exhaust gas, determining anoptimal rate of liquid flow required to reduce the TEG concentration insaid untreated gas to a target level and selectively controlling therate of liquid flow through said membrane array to match said optimalrate of liquid flow.

The method may include the further step of determining the effectivenessof said membrane array at reducing concentrations of said TEG bydetermining whether said liquid passing through said array experiencesone or both of a pressure drop that exceeds a predetermined level or apH drop that is less than a predetermined level.

The membrane array may comprise a module housed in a modular housingwherein said liquid is circulated through a selected number of saidmodules based on a determination of the level of TEG concentration insaid exhaust gas and/or the flow rate of said exhaust gas. Selected onesof said modules may be removed and replaced if it these have beendetermined to be less effective by a predetermined level.

According to another aspect, the invention relates to an apparatus forlowering the concentration of at least one target emission gas (TEG)from a source of engine exhaust gas comprising:

-   -   an enclosure for receiving a stream of engine exhaust    -   at least one array of hollow fiber ceramic membranes having a        bore and configured such that said exhaust contacts the        membranes as the exhaust gas is circulated through the array,        each of said membranes comprising a semi-permeable membrane wall        which is permeable to said TEG but non-permeable to non-TEG's in        said emission gas and a hollow bore;    -   a liquid inlet for feeding a carrier liquid into said membrane        bores in an unsaturated state;    -   a liquid outlet for receiving said carrier liquid from said        bores after circulation therethrough in a state saturated with        said TEG; and    -   a carrier liquid circulation subsystem to circulate said carrier        liquid through said membrane bores and said inlet and outlet        manifolds;    -   wherein said apparatus is configured wherein exhaust gas        circulated through said array contacts said membranes at on an        exterior surface of the membranes, said liquid contacts said        membranes on an opposed surface thereof and said TEG thereby        permeates through said membrane from the exterior membrane        surface into the bore to transfer said TEG from said exhaust gas        into said carrier liquid.

The apparatus may further comprise a carrier recycling subsystem incommunication with the primary carrier outlet and inlet, said recyclingsubsystem comprising a TEG stripping device for removing at least oneTEG from said carrier liquid, wherein said carrier is circulated in anessentially closed loop through said apparatus.

The carrier liquid may comprise water which is circulated in an openloop through said apparatus, said apparatus comprising a water inlet anda water outlet for non-recycling circulation of water through saidmembrane array.

The apparatus may comprise multiple ones of said membrane arraysarranged in parallel or in series for contacting the emission gas, foroperation in one of a parallel mode or a sequential mode of circulatingthe liquid.

According to a still further aspect, the invention relates to a systemfor lowering the concentration of at least one target emission gas (TEG)from a source of engine exhaust gas comprising:

-   -   an enclosure for receiving a stream of engine exhaust;    -   at least one gas treatment module for installation within said        enclosure, said module comprising a housing and an array of        hollow fiber membranes supported within the housing and having a        bore and configured such that said exhaust contacts the        membranes as the exhaust gas is circulated through the array        when the module is installed within the enclosure, each of said        membranes comprising a semi-permeable membrane wall which is        permeable to said TEG but non-permeable to non-TEG's in said        emission gas and a hollow bore;    -   a liquid inlet for feeding a carrier liquid into said membrane        bores in an unsaturated state;    -   a liquid outlet for receiving said carrier liquid from said        bores after circulation therethrough in a state saturated with        said TEG; and    -   a carrier liquid circulation subsystem to circulate said carrier        liquid through said membrane bores and said inlet and outlet        manifolds;    -   wherein said apparatus is configured wherein exhaust gas        circulated through said array contacts said membranes at on an        exterior surface of the membranes, said liquid contacts said        membranes on an opposed surface thereof and said TEG thereby        permeates through said membrane from the exterior membrane        surface into the bore to transfer said TEG from said exhaust gas        into said carrier liquid.

The system may further include a carrier recycling subsystem incommunication with the carrier liquid outlet and inlet, said recyclingsubsystem comprising a TEG stripping device for removing at least oneTEG from said carrier liquid, wherein said carrier is circulated in anessentially closed loop through said apparatus.

Alternatively, the carrier liquid comprises water which is circulated inan open loop through said apparatus, said apparatus comprising a waterinlet and a water outlet for non-recycling circulation of water throughsaid membrane array.

The modules may further comprise one or both of a liquid inlet manifoldor liquid outlet manifold in fluid communication with said bores atinlet and outlet ends of said bores respectively.

The system may further comprise at least one of a pH sensor system fordetermining a pH drop in said liquid carrier from circulating throughsaid membrane array and a pressure sensor system for determining apressure drop in said liquid carrier from circulating through saidmembrane array, said sensors being operatively linked to a signalprocessor for determining whether said pH drop and/or pressure drop isindicative of a reduced level of effectiveness of said membrane array atreducing concentrations of TEG.

The system may further comprise a sensor for measuring TEG concentrationwithin untreated exhaust gas from said source and a control system inoperative communication with said sensor and with a pump for controllingthe flow rate of said carrier liquid through said system, said controlsystem being configured to determine the flow rate of said carrierliquid through modules required in order to achieve a selected level ofTEG concentration reduction and to control said flow rate to providesaid flow rate.

The invention further relates to a kit comprising the apparatus orsystem as described herein and at least one carrier liquid fordissolving said TEG. The carrier liquid is one or more of an ionicliquid or sodium hydroxide. The ionic liquid may comprise one or moreof:

-   -   1,1,3,3-tetramethylguanidium lactate [TMG][L]    -   Monoethanolammonium lactate [MEA][L]    -   i-Butyl-3-methylimidazolium tetrafluoroborate [BMIm][BF₄]    -   i-Butyl-3-methylimidazolium methylsulfate [BMIm][MeS0₄]    -   i-Hexyl-3-methylimidazolium methylsulfate [HMIm][MeS0₄]    -   i-Ethyl-3-methylimidazolium methylsulfate [EMIm][MeS0₄]    -   i-Butyl-3-methylimidazolium hexafluorophosphate [BMIm][PF₆].

An ionic liquid, used in association with an appropriate semipermeablemembrane, can separate, capture and store a Target Emission Gas (TEG)such as SOX, NOX and/or COX from the exhaust gas in a closed loopreversible process. This alternative can eliminate or reduce theproduction of waste water and waste sludge in comparison with certainother solvents.

An ionic liquid (IL) is a solution that contains an organic cation (e.g.imidazolium, pyridinium, pyrrolidinium, phosphonium, ammonium), and apolyatomic inorganic anion (e.g. tetrafluoroborate, hexafluorophosphate,chloride) or an organic anion (e.g. trifluoromethylsulfonate,bis[(trifluoromethyl)sulfonyl]imide. The main advantages of ILs aretheir negligible volatility, non-flammability and good chemical andthermal stability. They are considered as environmental benign carriersas compared to volatile organic solvents, reducing the environmentalrisks of air pollution. Furthermore, certain properties of ILs(hydrophobicity, viscosity, solubility, acidity and basicity etc.) canbe tuned to improve the solubility of one or more TEGs within the IL byselecting a specific combination of cation and anion and varied byaltering the substitute group on the cation or the combined anion.

An ionic liquid may be “task specific.” An example of such a TaskSpecific Ionic Liquid (TSIL) is formed by the reaction of 1-butylimidazole with 3-bromopropylamine hydrobromide, following a workup andanion exchange. This yields an ionic liquid active at room temperature,incorporating a cation with an appended amine group. The ionic liquidreacts reversibly with CO₂ reversibly sequestering the gas as acarbamate salt. The ionic liquid, which can be repeatedly recycled, iscomparable in efficiency for CO₂ capture to commercial aminesequestering reagents and yet is nonvolatile and does not require waterto function. The unique properties of ionic liquids make themparticularly well-suited for physical and chemical absorption processes.They can be easily adjusted by substituting cations and anions in theirstructure and thereby “tuned” to absorb specific gases by eitherphysical and or chemical absorption over specified processing conditionsincluding temperature and pressure. These task specific ionic liquidsprovide significant improvements in chemical absorption efficienciesover other solvents

Ionic liquids have application in various liquid chemical separationprocesses. An example of an IL application is the BASIL (Biphasic AcidScavenging utilizing Ionic Liquids) process developed by BASF, in which1-alkylimidazole scavenges an acid from an existing process. ILcompounds are also used in chemical synthesis such as the synthesisprocess for 2,5-dihydrofuran by Eastman and the difasol process, anIL-based process which is a modification to the dimersol process bywhich short chain alkenes are branched into alkenes of higher molecularweight. A further IL-based process is the Ionikylation process developedby Petrochina for the alkylation of four-carbon olefins with isobutane.

The invention is based on the principle that SOX, NOX, and/or COX can beselectively removed from marine exhaust gases by the use of a liquidcarrier circulated through a semi-permeable membrane system such as aceramic membrane. These impurities are generally considered safe fordischarge when dissolved into a liquid but should not be discharged asgasses into the atmosphere. With the use of a membrane to separate suchcompounds, the TEG can permeate through the membranes while particulateswithin the marine exhaust including ash, soot, and oils do not. Thecarriers remain clean and devoid of toxic impurities, and can be safelydischarged, re-used, or regenerated.

The system according to the invention can be operated in an operatingmodes consisting of one of an Open Mode, a Closed Loop or a ZeroDischarge mode.

The liquid carrier used in an Open Mode can be the water within whichthe vessel floats, which can be fresh water or sea water. The membraneseparation system comprises an array of porous hollow fiber membranemembranes in which fresh water or sea water circulates within theinteriors of the membranes. The fresh water or sea water is drawn intothe vessel from surrounding waters and is circulated through the hollowfiber membrane membranes. Flue gases pass over and contact the exteriorof the porous hollow fiber membrane membranes and permeate through themembrane. One or more TEG's is absorbed by the water and removed fromthe exhaust stream. The absorbed gases form acids, which are neutralizedby the hardness of the fresh water or salinity of the sea water asprecipitates such as sulfides. The fresh water or sea water containingthe precipitates is subsequently discharged into the surrounding watersof the ship.

The carrier liquid used in a Closed Loop mode can be a basic solutionsuch as sodium hydroxide, which is circulated through a hollow fibermembrane array. Flue gases contact the porous hollow fiber membrane andpermeate through the membrane into the bore within which the carriercirculates. TEG's are absorbed by the solution within the membrane boreand thus removed from the exhaust stream. The absorbed gases form acidswhich are neutralized by the base. The heat absorbed by the carrierliquid as it passes through the membrane array elevates the carriertemperature and maintains the TEG compounds in solution. The carrierliquid can then be cooled within a desorption vessel, which causes theTEG compounds to precipitate in solid form such as sulfide precipitates.The precipitated solids can then be removed by a mechanical separationprocess such as filtering. The unsaturated carrier liquid can then berecirculated as a closed circulation loop. Cooling of the carrier liquidwithin the desorption vessel can be provided by use of a heat exchangedwithin the vessel in which ocean water is circulated as a cooling fluid.

The liquid carrier used in a Zero Discharge mode is an ionic liquid(IL). The zero discharge mode comprises a closed loop reversible processwhere little or no chemical precipitates are generated. The membraneseparation system comprises an array of porous hollow fiber membranemembranes through which IL circulates and a desorption vessel (DV) forseparating the TEG's from saturated IL. The sulfur dioxide, nitrogenoxides and carbon oxides can be separated from the ionic liquids withinthe DV by the application of one or more of differential pressure,temperature, and/or electric potential. The separated gases are thenstored in pure states or as compounds, and the ionic liquid reused. Theabsorbed gases are stored and be used for commercial applications. Thedifferential temperature required to dissociate the gases is provided bythe exhaust gases by means of a heat exchanger.

By means of the invention, exhaust gases permeate through the ceramicporous membranes but toxic particulates within the marine exhaustincluding ash, soot, and oils are too large to permeate through themembrane pores. The carriers remain clean and void of toxic impuritiesand can be safely discharged, re-used or regenerated in open loop,closed loop, or zero discharge modes. In contrast, conventional WetWater Scrubbers may spray carriers directly into the marine exhaust.Toxic particulates become trapped and suspended within the carriers, andmust be removed from the carriers using complex, energy intensive, andexpensive cleaning systems. The cleaning process produces a sludgebyproduct that is expensive to dispose of on land.

DEFINITIONS

In the present patent specification, the following terms shall have themeanings described below, unless otherwise specified or if the contextclearly requires otherwise:

“Gas” or “gasses” refer to a compound or mixture of compounds thatexists in the gas phase under ambient conditions of temperature andpressure.

“Diesel” refers to an internal combustion engine that of thecompression-ignition design. A diesel engine can burn a variety of fuelsincluding without limitation diesel fuel, bunker crude, biodiesel andothers. The term “diesel” or “diesel emissions” is not restricted to anyparticular fuel type but includes any hydrocarbon fuel that may becombusted in a diesel-type engine.

“Target Emission Gas” or “TEG” refers to any gas or gasses that areintended to be removed from an exhaust gas stream generated by acombustive process. TEG's can include but not limited to Sulfur Oxides,Nitrogen Oxides, and Carbon Oxides such as C0₂. It will be understoodthat a TEG can exist in either a gas phase or a liquid or solid phaseunder different conditions such as when dissolved into solution or boundto a liquid phase compound.

“Emissions” refers to total combustion exhaust gasses from an engine orother source of exhaust gasses, including target emission gas as well asother gasses.

“Carrier” refers to either one of a liquid containing a compound that iscapable of binding to a TEG or a liquid that can dissolve a TEG intosolution so as to be operative in a membrane system to selectivelyreduce the concentration of the TEG from a gas-rich environment.

“Semi-permeable membrane” may also be termed a selectively permeablemembrane, a partially permeable membrane or a differentially permeablemembrane, and is a membrane that allows selected molecules or ions topass through it by diffusion. The rate of passage through the membranecan depends on the pressure, concentration, and temperature of themolecules or solutes on either side, as well as the permeability of themembrane to each solute. The membrane can vary in thickness, dependingon the composition of the membrane and other factors.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand various exemplary embodiments, referenceis made to the accompanying drawings, wherein:

FIG. 1 is a schematic drawing showing an emissions reduction systemaccording to one embodiment of the invention;

FIG. 2 is a perspective view of a gas absorption module according to thepresent invention.

FIG. 3 is a perspective view, exploded, of the gas absorption module ofFIG. 2.

FIG. 4 is a cross-sectional view of a gas absorption module andassociated housing and gas duct components.

FIG. 5 is a schematic view of internal components of the gas absorptionmodule.

FIG. 6 is a schematic view of a hollow fiber ceramic membrane within agas absorption module, schematically showing selective absorption ofTEG's.

FIG. 7 is a schematic view a gas treatment system according to oneembodiment of the invention.

FIG. 8 is a schematic view a gas treatment system according to a secondembodiment of the invention.

FIG. 9 is a schematic view a gas treatment system according to a thirdembodiment of the invention.

FIG. 10 is a schematic view a gas treatment system according to a fourthembodiment of the invention.

FIG. 11 is a schematic view a gas treatment system according to a fifthembodiment of the invention.

FIG. 12 is a schematic view a gas treatment system according to a sixthembodiment of the invention.

FIG. 13 is a schematic view a gas treatment system according to anembodiment of the invention, showing in particular system control means.

FIG. 14 is flow chart showing operation of the control system accordingto one embodiment of the invention.

FIG. 15 is a graph showing the influence of water temperature on SOxabsorption rate within a gas absorption module of the invention.

FIG. 16 is a graph showing the influence of water flow rate through thehollow fiber membrane array on SOx absorption rate within a gasabsorption module of the invention.

FIG. 17 is a graph showing the influence of the exhaust gas flow ratio(actual flow/design flow rate) on SOx absorption rate within a gasabsorption module of the invention.

FIG. 18 is a schematic view of a gas desorption vessel according to afurther aspect of the invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of an embodiment of an exhaust gastreatment system 20 according to the invention, which is useful forreducing the concentration of one or more target emission gasses (TEG's)2 from an exhaust gas stream 1. Gas stream 1 comprises a mixture of TEGmolecules 2 and non-TEG molecules 3. The exhaust gas 1 may be generatedby a marine diesel engine or other combustion process. For example, thesystem may be adapted to process exhaust from a heater, a burner or agas turbine as well as various types of internal combustion engines. Thegas treatment system 20 shown in FIG. 1 is a “closed loop” system thatcomprises in general terms a gas absorption unit 22, a TEG desorptionunit 24 for separating the sequestered TEG compounds from the carrierliquid, and associated conduits, valves, pumps and other components forcirculating exhaust gas, carrier and separated TEG, as described below.In the embodiment of FIG. 1, gas treatment system 20 further comprises agas storage module 28 which stores the isolated TEG in the form ofcompressed gas or other suitable storage form. As discussed below, atleast some TEG's may be disposed of without storage, for example bydischarging into the ocean in an aqueous solution.

Gas absorption unit 22 comprises a main housing 30, seen in detail inFIG. 4, which houses one or more absorption modules 26. Exhaust gas iscirculated through main housing 30 from gas inlet plenum 32, whichreceives gas from engine conduit 34. The exhaust gas is circulatedthrough one or more absorption modules 26 that are mounted within mainhousing 30, following which the treated exhaust gas is exhausted throughoutlet plenum 36 into gas outlet conduit 38 for discharge into theenvironment.

Multiple modules 26 can be configured within main housing 30 in an arrayfor operation in parallel or in series for removing a selected TEG'sfrom the engine exhaust. Operation of system 20 in parallel refers to amode of operation wherein carrier is fed to multiple modules 26 inparallel, such that each module receives equally unsaturated carrierliquid. Operation of system 20 in series refers to a mode of operationwherein the carrier liquid is fed in series through multiple modules 26whereby the liquid becomes increasingly saturated as it passes throughthe respective modules. FIG. 1 depicts a system containing a singlemodule 26; FIGS. 7-12 depict alternative treatment systems in whichabsorption system 20 comprises multiple absorption modules 26. Eachabsorption module 26 contains therein a membrane assembly 66.

Exhaust gas enters gas absorption unit 22 through an inlet conduit 34and is discharged after treatment through outlet conduit 38. Unsaturatedliquid carrier is fed into gas absorption unit 22 through liquid inletconduit 40. The saturated liquid carrier exits unit 22 through outletconduit 42 and is then fed into desorption unit 24 where the TEG isremoved from the carrier. As discussed below, the carrier liquid absorbsone or more TEG's from the exhaust gas for transport to a separatelocation for storage or disposal. The now-unsaturated carrier is thenrecirculated into inlet conduit 40. As seen in FIG. 1, liquid flow ispressurized by a first pump 44 within outlet conduit 42 and a secondpump 172 within the inlet conduit 40. Gas outflow from desorption unit24 is pressurized by pump or compressor 46. A heat exchanger 48 isin-line with liquid conduit 40 to remove excess heat from the recycledcarrier. A coolant fluid (gas or liquid) enters heat exchanger 48through inlet conduit 49 and exits through outlet conduit 51, foroptional on-board use on the vessel.

As shown generally in FIG. 1, saturated carrier liquid from separationabsorption unit 22 enters desorption tank 24 wherein the saturatedcarrier is subjected to conditions of relatively reduced pressure and orincreased temperature. Under these conditions, the dissolved and/orbound TEG degasses and bubbles out. Dissolved mineral salts precipitateout of solution and settle to the bottom of the tank. The separated gasaccumulates at the top of tank 24, from where it is released through gasoutlet 25. The released gas from tank 24 flows through pipe 45 and ispressurized therein by gas pump 47, which pumps the TEG into one or morepressurized gas storage vessels 28 for safe disposal, either on-board toon shore. The now-unsaturated carrier is then piped back into absorptionunit 22 through inlet conduit 40.

Gas treatment system 20 further comprises a pH sensor 54 for measuringthe pH of carrier liquid within outlet conduit 42. System 20 furthercomprises a first pressure sensor 56 for measuring the carrier liquidpressure within inlet conduit 40 and a second pressure sensor 58 formeasuring carrier pressure within outlet conduit 42. One or more firstTEG sensors 60 are provided for detecting the level(s) of selected TEG'swithin the untreated exhaust entering system 20 within engine exhaustconduit 34. One or more second TEG sensors 62 are provided for detectingthe levels of the selected TEG's within the treated exhaust in dischargeconduit 38. The respective sensors 60 and 62 are in operativecommunication with a control system 200 whereby the values detectedthereby are transmitted in realtime to control system 200 for efficientoperation of the system, as described in more detail below.

As seen in more detail in FIGS. 2-5, gas absorption module 26 comprisesa housing 64 for housing a membrane assembly 66. Untreated exhaust gas 1enters housing 64 for contact with assembly 66, following which thescrubbed gas 3 exits housing 64. The scrubbed exhaust gas is at leastpartially depleted of one or more TEG's 3. Within housing 64, TEG's 3are stripped from the exhaust gas 1 by contact with a hollow fibersemi-permeable membrane using a carrier-based gas absorption process.Fresh (unsaturated) relatively cool carrier enters housing 64 throughcarrier inlet conduit 40 and saturated, TEG-laden carrier liquid 72exits through outlet conduit 42.

Module housing 64 can be modular in configuration to permit convenientassembly of multiple modules 26 in the form of a single unit forinstallation in a vessel or elsewhere. As discussed below, multiplemodules 26 can be linked in parallel or series depending on theapplication. In one example, housing 64 is rectangular and hasdimensions of 50 cm×50 cm×100 cm. Housing 64 may be fabricated frommetal sheeting such as a heavy gauge stainless steel sheet. Multiplemodules 26 can be secured in a rack for access and easy replacement.

Housing 64 is fabricated from sheet metal and comprises opposing sidewalls 74 a and 74 b and opposing end walls 76 a and 76 b. For purposesof description, an elongate axis “a” can be considered to extend betweenend walls 76 a and b. The interior of housing 64 is divided into twoessentially equal spaces by a central divider wall 78 which is parallelto end walls 76. Divider wall 78 supports hollow membrane membranes 80within housing 64, as described below. External bracing members 82 maybe provided for additional structural integrity of housing 64. Housing64 is open above and below to allow gas to flow freely through thehousing.

Housing 64 retains within its interior first and second perforated walls84 a and 84 b (seen in FIG. 3), each having an array of perforations 86.Perforated walls 84 a and b are secured to corresponding end walls 76 aand b, and are of essentially identical configuration thereto tosubstantially cover the respective end walls 76.

End walls 76 a and 76 b have recessed central portions 88 a and 88 brespectively that open to the interior of housing 64. Recesses 88 a andb are covered by respective perforated walls 84 a and b, which aresealed and secured to end walls 76 by mounting strips 85 and gaskets 87.Recesses 88 a and b each define an enclosed manifold, recess 88 bdefines an inlet manifold and recess 88 a defines an outlet manifold.

Perforated walls 84 may be secured to end walls 76 by bolts or otherfasteners.

Housing 64 houses within its interior one or more membrane assemblies66. Each assembly 66 consists of an array of porous ceramic hollow fibermembranes 80 that span the interior of housing 64, extending axiallybetween end walls 76 a and b. Membranes 80, one of which is shown indetail in FIG. 6, each comprise a tubular ceramic membrane wall 90 and ahollow central bore 92. In operation, shown schematically in FIG. 5,liquid carrier flows through bore 92 while exhaust gas contacts theexterior of membrane wall 90. Membranes 80 are semi-permeable in thatthe membrane wall has pores that permit TEG's to permeate the wall intothe bore, while other exhaust gasses are blocked. The liquid carriercirculating within bore 92 is unable to penetrate membrane wall 90. Theflow of unsaturated carrier through bore 92 maintains a lower gaspartial pressure of TEG's within the carrier, thereby generating a flowof TEG across membrane wall 90 from the gas side, where the partialpressure is relatively high, to the carrier side where the partialpressure is low. As a result, membranes 80 are able to separate TEG'sfrom an exhaust gas stream channeled through housing 64.

Suitable ceramic hollow fiber membranes include commercially availablealuminum oxide (AI2O3) hollow fibre membranes, such as the Membralox®membrane. A description of this membrane is available at:http://www.pall.com/main/food-and-beverage/product.page?id=41052.Representative dimensions of a suitable membrane 80 is: pore size: 100A;ID: 4 mm; length: 1020 mm.

Opposing ends of membranes 80 are secured within openings 86 in walls 84a and b. Membrane bore 92 communicates with a respective opening 86 ateither end of membrane 80. The intersection between membrane 140 andeach corresponding opening 86 is sealed against fluid (gas and orliquid) leakage. For example, membranes 80 may be secured to walls 84 atopenings 86 by a soldering or gluing process. Membranes 80 pass throughopenings 94 within divider wall 78, which supports membranes 80 at theirmidpoint. It will thus be seen that fluid entering into inlet manifold88 b is distributed across membrane array 96 wherein the fluid entersinto bores 92 of membranes 80. The carrier then flows through bores 92and is discharged into outlet manifold 88 a. All liquid-filled spaceswithin housing 64 are sealed against leakage.

Unsaturated carrier liquid enters inlet manifold 88 b through liquidinlet 98 (seen in FIG. 3) from where it is distributed into membranes80. After passing through membrane array 96, the now-saturated carrierenters outlet manifold 88 a from where it is discharged through outlettoo. Inlet 98 and outlet 100 are connected to hoses or other liquidconduits, shown schematically in FIGS. 1-3, leading to other componentsof system 3.

Untreated exhaust gas enters housing 64 through inlet plenum 32, whichdischarges untreated (raw) exhaust gas from an engine or other source ofcontaminated gasses that contains a TEG. The gas flows through theinterior of housing 64, contacting membrane array 96 as the gas travelsto outlet plenum 36. Membrane array 96 essentially fills the interior ofhousing 64 whereby a large portion of the gas contacts at least onemembrane wall 90 as the gas flows through the housing. The amount ofcontact between exhaust gas and the membrane surfaces will be determinedby several factors including the configuration of array 96, the size andspacing of membranes 80 and the speed of gas flow through housing 64.Increased contact may be obtained by closer spacing of membranes and alarger number thereof, although this has to be balanced against apossible increase of backpressure and other factors. As a result, theconfiguration of membrane array 96 including the number of tubularmembranes that can be included within a housing of a given size, willdepend to some extent on the parameters of the engine that provides theexpected source of emissions and such factors as the backpressure thatcan be imposed by device 3 without causing significant decrease inengine performance.

The respective gas and carrier flowpaths through the housing 64, whereinthe gas and liquid streams contact opposing surfaces of membranes 80,are shown schematically in FIGS. 5 and 6. As shown, liquid 72 flowsthrough the bore 92 or membrane 80 while the emission gas 1 contacts theexterior of membrane 80. As the raw emission gasses 1 contact thesurface of membrane 80, the TEG molecules 68 within gas 1 permeatethrough membrane 80 from a region of high gas concentration (high gaspartial pressure) to a region of low gas concentration (low gas partialpressure). Non-TEG molecules 147 are excluded from membrane 80 and thusconcentrate within housing 64 exteriorly of membranes 80, to form aconcentrated emissions gas that is rich in non-TEG components andcontaining a reduced amount of TEG.

The exterior of membranes 80 thus consists of a high partial pressureside of membrane wall 90, in which the partial pressure of TEG's withinthe exhaust gas is relatively high in comparison with the partialpressure of the carrier circulating within bore 92. The difference inpartial pressure drives the TEG's from the exterior to the interior ofmembrane 80. Carrier 72 flows through the interiors of membranes 80 tomaintain a consistently low gas partial pressure of the TEG's.

TEG molecules 68 diffuse through the membrane according to Fick's law ofdiffusion and exit the membrane material at the low pressure side, wherethey dissolve into the permeate liquid 72 or otherwise combine withliquid 72. The stripped exhaust gas, which is rich in non-TEG molecules3 and low in TEG molecules 68, then exits housing 64 for discharge intothe atmosphere.

Carrier liquid 72, carrying TEG's 68 in dissolved or bound form(depending on the carrier), then exits housing 64 and is circulated togas desorption vessel 24. Desorption vessel 24 is depicted schematicallyin FIG. 19 Vessel 24 comprises a tank for retaining the IL therein, andcomprises an inlet 102 for gas-bearing IL, a liquid outlet 104 for therecycled (non-gas bearing) IL and a gas outlet 25 for discharge of gasseparated from the ionic liquid, into gas conduit 108. The tank maycomprise a tank wall of stainless steel or low carbon steel. Thepressure within the tank is reduced relative to the fluid pressurewithin the conduits. Tank 24 is also maintained at an elevatedtemperature via a heat exchanger. Heating fluid enters inlet 360 andexits outlet 361. Ionic liquid enters tank 24 through inlet 102 and isallowed to degas within the tank. Within desorption vessel 24, TEG's(such as SOX, NOX, or COX) that have dissolved into the ionic liquiddegas and are released from solution as bubbles under conditions ofreduced pressure and/or elevated temperature relative to theseconditions within absorption module 26. Optionally, an electric chargecan be applied within vessel 24 to improve the efficiency of the gasseparation step. The released gasses accumulate in tank 24 at an upperregion above liquid inlet 102. The separated gases are released from gasoutlet 25. The discharged gasses are then pressurized by compressor 46for storage within gas storage tank 28. The compressed gasses may thenbe safely disposed of on land. The IL is cooled via heat exchanger priorto discharge from outlet 104 and re-use. Coolant fluid enters inlet 362and exits outlet 363. Precipitation of salts and insoluble compoundswithin Tank 24 settle in the bottom and can be periodically purged viavalve 365.

Carrier liquid 72 may comprise a task specific ionic liquid (TSIL) whichbinds with the TEGs molecules and increases diffusion efficiency throughthe phenomenon commonly referred to as the facilitated transport.

Examples of TSILs that may be used in the present invention, eitheralone or in combination, include:

-   -   1,1,3,3-tetramethylguanidium lactate [TMG][L]    -   Monoethanolammonium lactate [MELA][L]    -   i-Butyl-3-methylimidazolium tetrafluoroborate [BMIm][BF₄]    -   i-Butyl-3-methylimidazolium methylsulfate [BMIm][MeS0₄]    -   i-Hexyl-3-methylimidazolium methylsulfate [HMIm][MeS0₄]    -   hyl-3-methylimidazolium methylsulfate [EMIm][MeS0₄]    -   i-Butyl-3-methylimidazolium hexafluorophosphate [BMIm][PF6])    -   tyl-3-methylimidazolium trifluoromethanesulfonate [BMIM]OTf .    -   i-butyl-3-methyl-imidazolium hexafluorophosphate ([C4mim][PF₆])

Alternatively, carrier 150 may comprise sodium hydroxide, which can beused to absorb sulfur oxides from the emission stream and neutralizesulfur acids.

FIGS. 7-12 depict alternative embodiments of gas treatment system 20.

One embodiment of system 20, seen in FIG. 7, is an “open” systeminstalled in a marine vessel 300. In this embodiment, carrier liquid 72comprises water such as sea water or fresh water pumped from thesurrounding water environment of the vessel and then discharged backinto the water after one or more TEG compounds have dissolved into thewater. Water (in particular seawater) can absorb sulfur oxides from theemission stream and neutralize sulfur acids. Gasses generated by marinediesel engine 302 are discharged into exhaust conduit 34. Conduit 34opens to an absorption unit 22 via inlet manifold 32. Within desorptionunit 22 are installed multiple (in this case four) gas absorptionmodules 26 a-d, which are linearly arranged in series within housing 30.Exhaust gas passes through housing 30, contacting respective membraneassemblies 66 within modules 26 a-d and is discharged to the atmospherethrough discharge conduit 38.

In the embodiment of FIG. 7, seawater (or freshwater, if the vessel istraveling in a freshwater environment) is drawn from the surroundingwater through inlet pipe 304, which opens at one end to the exterior ofvessel 300. Pipe 304 enters a pipe splitter 306 wherein the water flowis diverted through 4 individual pipes 308 a-d, which in turn each feedinto a corresponding inlet manifolds of respective absorption modules 26a, 26 b, 26 c and 26 d. Modules 26 a-d operate in parallel with respectto carrier circulation wherein the carrier is fed through the respectivemodules in parallel. The sea or fresh water circulates through therespective modules where it becomes saturated with TEG's dissolvedtherein from the exhaust passing through the respective modules. Thesaturated water is then collected into a common discharge conduit 310and is discharged back into the ocean. Water is pumped through thesystem by a pump 312 at the outlet end of the water circulation system.Pump 312 is controlled by pump controller 314, as discussed below.

The multiple modules can be the same or different. In the case ofdifferent modules, the membrane assemblies therein can be configuredwith different pore sizes and/or membrane wall thicknesses to absorbdifferent TEG's.

Furthermore, although FIG. 7 depicts four modules 26 a-d, any number ofmodules may be provided depending on the flow rate of exhaust gas,desired TEG reduction level and other parameters.

An embodiment depicted in FIG. 8 is an “open” system similar to FIG. 7.However, rather than a parallel delivery of carrier to modules 26 a-d,in the example of FIG. 8, carrier (sea/freshwater) is delivered tomodules 26 a-d in series, i.e. sequentially. Thus, water inlet conduit304 initially delivers water to module 26 a, from where it is dischargedinto module 26 b and so forth until finally discharged from module 26 d,back into the surrounding seawater. FIG. 8 depicts an optional componentthat dispenses a neutralizing compound such as MgOH which can beselectively introduced into the saturated seawater prior to dischargeinto the ocean to reduce the acidity of the discharged water in order tocomply with any applicable regulatory restrictions against discharge ofacid solutions. A basic solution is stored in a tank 316 and dischargedthrough a pipe 318 into water conduit 310. The basic solution is pumpedby a pump 320 which is controlled by controller 200 responsive to the pHlevel with the saturated water, as detected by pH sensor 54. The basicsolution is combined with the saturated carrier liquid at a rateselected to reduce the acidity therein by a selected level, for examplefor regulatory compliance.

FIG. 9 depicts a “closed loop” version of system 20 wherein the carrierliquid 72 consists of a fifty percent (50%) V:V NaOH:water solutionwhich is cycled through system 20. In this embodiment, engine exhaust ischanneled through gas absorption unit 22 which in this example comprisesfour TEG absorption modules 26 a-d. Unsaturated carrier liquid fromdesorption vessel 24 is pumped through absorption unit 22 by variablespeed pump 44 and circulated sequentially through modules 26 a-d. Pump44 is in turn controlled by a pump controller in operative communicationwith controller 200. Within absorption unit 22, the heat from the engineexhaust 1 elevates the temperature of the carrier liquid and causes itto absorb TEG compounds 68 such as sulfur oxides, which dissolve intosolution within carrier liquid 72. The acidic sulfur oxide molecules areneutralized within the sodium hydroxide carrier solution. Withindesorption vessel 24, the carrier liquid 72 is cooled, which causes thedissolved TEG's to precipitate out as solid precipitates 322. If the TEGcomprises sulfur oxides, the precipitates comprise sulfides. Theprecipitates 322 accumulate in the bottom of vessel 24 and can beremoved periodically for on-shore disposal. The cooling of carrierliquid 72 within desorption vessel 24 may be performed by a heatexchanger 324. Water from the surrounding environment is circulatedthrough heat exchanger 324 by pump 325, through water pipes 326. Pump325 is controlled by pump controller 328, which is in operativecommunication with controller 200.

FIG. 10 depicts an embodiment of system 20 wherein unsaturated carrierliquid 72 is pressurized by pump 46 and enters absorption unit 22through inlet conduit 40. The carrier flows in sequence through multipleabsorption modules 26 a-d. The now-saturated carrier then flows throughdischarge conduit 42 where it is pressurized by pump 44 and enters intodesorption vessel 24. Within desorption vessel 24, the carrier liquid issubjected to conditions whereby the absorbed TEG compounds 68 degas fromliquid 72, for example by reducing the pressure within vessel 24. Theseparated TEG compounds 68 are released in a gas phase through opening25 of vessel 24 into conduit 45. The TEG gasses are pressurized bycompressor 47 into storage vessel 28. The unsaturated carrier is thenpumped back into absorption unit 22 through inlet conduit 40. Theembodiment of FIG. 10 is configured to operate in a “zero discharge”mode, wherein the circulating carrier liquid can be an ionic liquid.

FIG. 11 depicts an embodiment similar to FIG. 10, with two absorptionmodules 26 a and 26 b. Carrier liquid becomes saturated within modules26 a and 26 b. The saturated carrier liquid is piped via conduit 42 intodesorption vessel 24 where it is de-gassed by means of de-pressurizingthe liquid. The unsaturated liquid is recirculated through modules 26 aand 26 b via conduit 40. In this embodiment, a single pump 46 isprovided to circulate the carrier liquid through the system anddegassing of the saturated carrier liquid is performed solely bydepressurizing liquid within vessel 24.

FIG. 12 depicts an embodiment of system 20 configured to independentlyseparate and store multiple selected TEG's in a zero discharge modewherein the selected TEG's are independently removed and stored. In thisembodiment, absorption unit comprises 6 absorption modules, 26 a-f. Themodules are arranged in three pairs, 26 a and 26 b being a first pair,26 c and 26 d being a second pair, and so forth. Each pair of modules isconfigured to channel carrier in series through the respective modulesof the pair. Different carrier liquids are circulated through therespective pairs of modules in independent circuits to individuallyseparate selected TEG's. A first closed carrier liquid loop comprises afirst carrier inlet 40 a which circulates carrier through modules 26 aand 26 b. The saturated carrier from the first loop is then dischargedinto discharge conduit 42 a into first desorption vessel 24 a. Withinvessel 24 a, a first TEG 68 a is separated from the carrier liquid andis pressurized into first gas storage vessel 28 a. A second closed loopcomprises conduits 40 b and 42 b, which circulate unsaturated carrierthrough a second pair of modules 26 c and d and a second desorptionvessel 24 b. A second gas storage vessel 28 b is provided to store asecond TEG 68 b. A third closed loop is similar in configuration forseparating and storing a third TEG 68 c. Carrier liquid 72 flows back tomodules 26 a-f through pipes 42 a-c to complete the three independentfluid circuits. The respective carrier liquids may comprise threedifferent ionic liquids, selected to absorb specific TEG's. For example,the carrier liquids may comprise: 1) i-Butyl-3-methylimidazoliummethylsulfate [BMIm][MeS04] for absorbing SOx, 2)i-butyl-3-methyl-imidazolium hexafluorophosphate ([C4mim][PF6]) forabsorbing CO₂, and 3) i-Butyl-3-methylimadazoliumtrifluoromethanesulfonate [BMIM]OTf for absorbing NOx.

A further alternative embodiment of a TEG desorption system is shown inFIG. 18. In this embodiment, saturated carrier liquid enters adesorption chamber 24 through inlet conduit 102 which outlets intovessel 24 at an upper portion thereof. A pressure drop on enteringchamber 24 causes the liquid to degas to release the TEG's. Thegas-phase TEG's are then discharged through conduit 25 and are pumped bycompressor 46 through conduit 108 into storage vessel 28. The liquidwithin chamber 24 is cooled by circulating a coolant fluid through asealed pipe within chamber 25. The coolant fluid enters via pipe 360 andis discharged by pipe 361. Unsaturated carrier liquid exits chamber 24adjacent its base, and enters into a secondary vessel. The carrierliquid is further cooled within the secondary vessel by additionalcoolant fluid which is circulated through a sealed pipe within theinterior of the secondary vessel. The additional coolant enters via pipe362 and exits via pipe 363. The cooled carrier liquid then exits thesecondary vessel through discharge conduit 104, for circulation withinone or more gas absorption modules 26, not shown.

The carrier used in the “zero discharge mode” embodiments may be a TaskSpecific Ionic Liquid “TSIL”. The TSIL comprises a reversible carrier.This permits the TEG+TSIL solution 7 (IL with TEG dissolved therein) tobe separated in the desorption vessels 28 a-c by the application ofdifferential pressure, temperature and/or or electric potential.

Treatment system 20 is normally able to operate at engine pressure. Insome cases, system 20 can generate excessive back pressure, depending onthe engine design or manufacturer-imposed requirements and the number ofother systems that contribute to back pressure such as turbo units, heatexchangers, pipe bends etc. If the back pressure exceeds a predeterminedmaximum, a booster fan 10 can be provided to boost the exhaust pressureupstream of system 20 to reduce back pressure imposed by system 20.

In one embodiment, heat from the engine exhaust is extracted with a heatexchanger prior to entering housing 64. This provides two benefits. Thefirst is that temperature of the marine exhaust is lowered to within thelower operating temperatures of certain polymer membranes and TSILs. Thesecond benefit is to apply the captured heat energy to provide thedifferential temperature to dissociate the TEGs+TSILs. The overallthermal efficiency of the system is improved, reducing the energy tooperate the system.

The desorption vessel 24 is operated at near vacuum pressure to improvethe dissociation rate of the TEGs and TSILs. An electric potential mayalso be applied to improve the dissociation of the TEGs and TSILs.

The TEGs are freed as a gas within the desorption vessels 24 a-c, andcollected and stored in a pressurized vessels 28 a-c, or combined as acompound for storage as a solid. The TSILs remains as a liquid withinthe desorption vessels 24 a-c. The TSIL is then pumped back to the gasabsorption unit 22.

A supplemental amount of TSIL may be added periodically from a storagevessel to replace any TSIL lost through evaporation or chemicaldecomposition.

As shown schematically in FIG. 13, absorption system 20 comprisesmonitors and detectors described below that monitor selected systemoperating parameters and transmit the resulting data to controller 200during operation of the system. These include: an upstream liquidpressure detector 56 which is measures carrier pressure prior to entryinto membrane modules 26; multiple downstream liquid pressure detectors58 which measure carrier pressure downstream of each membraneassemblies, wherein the detected difference between pressures representsa pressure drop occurring largely within a respective membrane assembly66; and multiple pH sensors 54 located downstream of respective membraneassemblies 66 for measuring the pH of carrier exiting each membranemodule 26. Optionally, a pH sensor can be provided upstream of membranemodules 26 to detect the pH level of the carrier liquid prior to flowingthrough the membrane modules 26 thereby allowing a determination of thepH difference.

The control system 200 for operation of gas treatment system 20 isdescribed below. The operation of system 20 is configure to optimize themass transfer or absorption exhaust gas to ensure that the exhaust gassufficiently contacts the membrane exterior surface to permit it to beabsorbed through the membrane, utilizing principles of mass transfer, orHenry's Law. Control system 200 comprises in general terms a computerprocessor that includes a random access memory (RAM), a data storagemodule such as a hard drive and a user interface 330 comprising displayand a data entry terminal. Control system 200 is in operativecommunication via wireless or wired data communication links with thesensors and detectors described herein and the various controllablecomponents described herein including the adjustable valves, pumps,compressors and other adjustable components described herein that permitoperation of gas treatment system 20.

As seen in FIG. 13, multiple pH sensors 54 and pressure sensors 58 areprovided within respective carrier discharge conduits 42. pH sensor 54transmits data to pH signal processor 350 and pressure sensor 58transmits data to pressure signal processor 352. The respective signalprocessors can comprise independent units in communication withcontroller 200 or incorporated therein. Carrier liquid valves 332 a-dare provided within respective carrier inlet conduits 40 to controlcarrier flow into respective absorption modules 26 a-d. Valves 332 a-dare independently controlled by a servomotor value controller 354. A TEGlevel sensor 62 is provided within exhaust discharge conduit 38 todetect the level(s) of selected TEG's. A TEG signal processor 356 isresponsive to signals generated by TEG level sensors 62. A pump motorcontroller 334 is associated with water pump 44 to control operation ofpump 44. The above detectors, sensors and controllers and operationallylinked to the main processor of control system 200, which in turn isoperationally linked to a user interface 356 via a system bus 336.

FIG. 14 is a flowchart showing operation of control system 200. In thisfigure:

TEGc=Target Emission Gas Concentration as measured with sensor 62 at thefunnel (exhaust outlet) after passing through the absorption unit 22.

TEGa=Target Emission Gas allowable limit, for example 25 ppm for SOX.

X=index for the counter, which tracks the numbers of gas absorptionmodules 26 that are in operation and non-operative.

N=total number of modules 26 available for use in system 20, for exampleN=20 modules for 8 MW engine.

Control system 200 operates initializes operation of the system andmonitors the performance of absorption modules 20 according to thefollowing steps:

1. At step 400, power-on control system 200 from standby mode. This stepmay be taken either before or after the vessel engine is powered on.

2. At step 402, enter into control system 200 form the user interfacethe total number of gas absorption modules 26 available in the system.This step may be pre-programmed into the control system. If notpreviously performed, the normal operating pressure of modules 26 mayalso be entered.

3. At step 404, measure the TEGc with gas sensor 62 and compare thisvalue to the TEGa at step 406. Step 406 further comprises adetermination of the number of modules of system 20 that should beactuated for system 20 to operate at an optimal efficiency level. Forexample, the system may contain 20 modules, and control system 200 maydetermine that only 15 modules are required to provide the target TEGreduction.

4. If the untreated engine exhaust contains a low level of TEG's below aselected value (TEGc is less than TEGa), the system will not turn on andthe system returns to standby mode at step 408. If the TEGc levelsexceed the TECa value, the system is put into operation at 410.

5. If the system is put into operation, liquid flow valve 332 a for afirst module 26 a is actuated at 412 and the liquid pump 44 is actuatedat step 414 to run at 1/N speed. This provides variable speed control.For example, if the system contains 20 modules, and control system 200determines that only 15 modules are required to provide the target TEGreduction, then pump 44 is run at 15/20 of full operational speed,thereby reducing the power requirements for operating the system. Thesystem then performs tests on the selected number of modules accordingto the steps described below. Pumps 312 are controlled by pumpcontroller 314 which is a unit that is either responsive to controller200 or incorporated therein.

6. The pH of the liquid solution is measured at the exit of the firstabsorption module 26 a by pH sensor 54 at step 416 . This value isindicated as pHx in FIG. 14. This pH level is compared to apredetermined value at step 418. When acidic gases such as SOX, NOX, COXare extracted into the liquid, this acidifies the liquid circulatingthrough the membranes. The level of acidification is used to determinewhether the membrane assembly has become fouled and incapable ofabsorbing TEG's wherein a pH drop that exceeds a target level (pHt) isindicative of fully functional membranes and a pH drop that fails toexceed this level is indicative of a membrane assembly that has becomefouled. This can avoid the need to visually inspect the membranes. Ifthe pH difference is less than 0.1 across a module, this is indicativethat acidic gases are not being absorbed by the modules 26 and themembranes therein are fouled. For reference, seawater pH is typicallylimited to a range between 7.5 and 8.4.

7. If pH X fails to reach pHt, indicative of fouling of membraneassembly 66 a, then valve 332 a is turned off at step 420, shutting offthe unit, and the SERVICE REQUIRED indicator 426 is actuated at step422. This sends a signal to service the affected module. Optionally, thesignal may be sent to both an on-board monitor and also a wirelesslytransmitted signal to an on-shore operator who can then arrange for areplacement module at the next port of call of the vessel. If the pHdetected at step 416 remains less than pHt, then the system proceeds tostep 424.

8. At step 424, carrier pressure is measured at the membrane outlet side(Px) within carrier discharge conduit. At step 425, this pressure iscompared with the input pressure detected by pressure sensor 56 todetermine a pressure drop. A pressure drop that exceeds a predeterminedlevel (pressure tolerance level, Pt) is indicative of a leak, forexample caused by a broken tube or seal.

9. If there is a leak, or broken tube, the control system will close thevalve at step 428 and sound an alarm at step 430. This can send asatellite signal to the next port of call to schedule service to thesystem.

10. If no excessive pressure drop is detected, the above steps arerepeated for subsequent modules 26 b, c etc. (X=X+i) at steps 432 and434 to determine whether any of these modules are fouled or leaking.Once the above steps have been performed for the optimal number ofmodules required for operation at the target efficiency, as determinedat step 406, controller 200 continues to run the system, as shown atstep 408, with this number of modules and at the corresponding pumpspeed for optimum efficiency.

Tests have been performed to show operational results obtained with thepresent system. The results of such tests are summarized in the graphsdescribed below.

FIG. 15 shows the effect of water carrier temperature on absorption rateof SOX. A lower water temperature increases absorption rate.

FIG. 16 shows the effect of water (carrier) flow rate on the absorptionrate of SOX. A faster flow rate increases absorption rate.

FIG. 17 shows the relationship between exhaust gas flow and absorptionrate of SOX. The efficiency drops as the flow rate increases above thepredetermined “design” flow rate.

The invention is not intended to be limited to the embodiments describedherein, but rather the invention is intended to be applied widely withinthe scope of the inventive concept as defined in the specification as awhole including the appended claims.

1. A method for reducing the concentration of a target emission gas(TEG) from a source of marine engine exhaust gas comprising the stepsof: directing said engine exhaust gas from the source into an enclosedspace containing at least one array of hollow fibre semi-permeableceramic membranes, wherein said exhaust gas contacts an exterior surfaceof said membranes whereupon TEG within said exhaust gas permeatesthrough said membrane thereby lowering the concentration of said TEGwithin said exhaust gas; circulating a carrier liquid capable ofretaining TEG compounds through bores of said hollow fibre ceramicmembranes thereby elevating the concentration of said TEG compoundswithin said carrier liquid; discharging said exhaust gas containing areduced TEG concentration from the enclosed space and removing saidcarrier liquid containing said TEG compounds therein from said hollowfibre ceramic membrane array.
 2. The method of claim 1 wherein aftersaid carrier liquid is removed from the membrane array, said TEGcompounds are separated from said carrier liquid to reduce theconcentration thereof in said carrier liquid and said carrier liquid isthen recycled back through said membrane array.
 3. The method of claim 1wherein said carrier liquid comprises an ionic liquid.
 4. The method ofclaim 3 wherein the carrier liquid is a task-specific ionic liquidspecific to said TEG.
 5. The method of claim 4 wherein the engineexhaust gas from the source enters the enclosed space at enginepressure.
 6. The method of claim 5 further comprising the step ofseparating said TEG compounds from said carrier liquid for storage andrecycling said carrier liquid through said membrane array.
 7. The methodof claim 4 wherein said ionic liquid is one or more of:1,1,3,3-tetramethylguanidium lactate [TMG][L]; Monoethanolammoniumlactate [MEA][L]; 1-Butyl-3-methylimidazolium tetrafluoroborate[BMIm][BF4]; 1-Butyl-3-methylimidazolium methylsulfate [BMIm][MeSO4];1-Hexyl-3-methylimidazolium methylsulfate [HMIm][MeSO4];1-Ethyl-3-methylimidazolium methylsulfate [EMIm/][MeSO4];1-Butyl-3-methylimidazolium hexafluorophosphate [BMIm][PF6];1-Butyl-3-methylimidazolium trifluoromethanesulfonate [BMIM]OTf; or1-Butyl-3-methylimidazolium hexafluoruphosphate ([C4mim][PF6]). 8-15.(canceled)
 16. A system for lowering the concentration of at least onetarget emission gas (TEG) from a source of engine exhaust gascomprising: an enclosure for receiving a stream of engine exhaust; aplurality of gas treatment modules configured for installation withinsaid enclosure, each of said modules comprising a housing and an arrayof hollow fibre ceramic membranes supported within the housing andconfigured so that said exhaust contacts the membranes as the exhaustgas is circulated through the array when the module is installed withinthe enclosure, each of said ceramic membranes comprising asemi-permeable membrane wall which is permeable to said TEG butnon-permeable to non-TEG's in said emission gas and a hollow bore; aliquid inlet for feeding a carrier liquid into said membrane bores in anunsaturated state; a liquid outlet for receiving said carrier liquidfrom said bores after circulation therethrough in a state saturated withsaid TEG; and a carrier liquid circulation subsystem to circulate saidcarrier liquid through said membrane bores and said liquid inlet andliquid outlet; wherein said apparatus is configured so that exhaust gascirculates at engine pressure through said array and contacts saidmembranes on an exterior surface of the membranes, said liquid contactssaid membranes on an opposed surface thereof and said TEG therebypermeates through said membrane from the exterior membrane surface intothe bore to transfer said TEG from said TEG compounds from said exhaustgas into said carrier liquid.
 17. The system of claim 16 furthercomprising a carrier recycling subsystem in communication with thecarrier liquid outlet and liquid inlet, said recycling subsystemcomprising components for removing at least one TEG from said carrierliquid wherein said carrier is circulated in an essentially closed loopthrough said apparatus.
 18. The system of claim 17 further comprising atleast one of pH sensor system for determining a pH drop in said liquidcarrier from circulating through said membrane array and a pressuresensor system for determining a pressure drop in said liquid carrierfrom circulating through said membrane array, said sensors beingoperatively linked to a signal processor for determining whether said pHdrop and/or pressure drop is indicative of a reduced level ofeffectiveness of said membrane array at reducing concentrations of TEG.19. The system of claim 17 further comprising a sensor for measuring TEGconcentration within untreated exhaust gas from said source and acontrol system in operative communication with said sensor and with apump for controlling the flow rate of said carrier liquid through saidsystem, said control system being configured to determine the flow rateof said carrier liquid required in order to achieve a selected level ofTEG concentration reduction and to control said pump to provide saidflow rate.
 20. The system of claim 17 further comprising a heatexchanger configured to lower the temperature of the engine exhaust gasbefore it enters the first of said plurality of gas treatment modules.21. The system of claim 20 where the apparatus is configured so that theheat from said heat exchanger is used in the carrier recyclingsubsystem.
 22. A kit comprising the system of claim 17 and at least onecarrier liquid for dissolving said TEG.
 23. The kit of claim 22 whereinthe carrier liquid is one or more of an ionic liquid or sodiumhydroxide.
 24. The kit of claim 23 wherein said ionic liquid comprisesone or more of: 1,1,3,3-tetramethylguanidium lactate [TMG][L];Monoethanolammonium lactate [MEA][L]; 1-Butyl-3-methylimidazoliumtetrafluoroborate [BMIm][BF4]; 1-Butyl-3-methylimidazolium methylsulfate[BMIm][MeSO4]; 1-Hexyl-3-methylimidazolium methylsulfate [HMIm][MeSO4];1-Ethyl-3-methylimidazolium methylsulfate [EMIm][MeSO4];1-Butyl-3-methylimidazolium hexafluorophosphate [BMIm][PF6];1-Butyl-3-methylimidazolium trifluoromethanesulfonate [BMIM]OTf; or1-Butyl-3-methylimidazolium hexafluouphosphate ([C4mim][PF6]).
 25. Anapparatus for lowering the concentration of sulphur oxides (SOX) fromuntreated marine diesel engine exhaust gas comprising: an enclosure forreceiving a stream of untreated engine exhaust at engine pressure havinga gas inlet for receiving said exhaust gas containing said SOX's and agas outlet for discharging said exhaust gases; at least one array ofhollow fibre ceramic membranes within the enclosure for reducing theconcentration of said SOX's within the exhaust gas configured wherebysaid exhaust gas contacts the membranes when circulated through themembrane array, each of said membranes comprising a semi-permeablemembrane wall which is permeable to said SOX in said emission gas and ahollow bore; a liquid inlet for feeding an ionic liquid into saidmembrane bores; a liquid outlet for receiving said ionic liquid fromsaid bores after circulation therethrough; a liquid circulationsubsystem to circulate said ionic liquid through said membrane bores fordischarge or recycling through said membrane array; an ionic liquidrecycling subsystem in communication with the liquid outlet and inletand comprising components for removing SOX from said ionic liquid,wherein said ionic liquid is circulated in an essentially closed loopthrough said apparatus; and a sensor for measuring SOX concentrationwithin untreated exhaust gas and a control system in operativecommunication with said sensor and with a pump for controlling the flowrate of said carrier liquid through said system, said control systembeing configured to determine the flow rate of said carrier liquidrequired in order to achieve a selected level of SOX concentrationreduction and to control said pump to provide said flow rate.
 26. Thesystem of claim 25 further comprising a heat exchanger configured tolower the temperature of the engine exhaust gas before it enters theenclosure. 27-28. (canceled)
 29. A method for reducing the concentrationof a target emission gas (TEG) from a source of marine engine exhaustgas comprising the steps of: Directing said engine exhaust gascontaining TEG from the source into an enclosed space containing atleast one array of hollow fibre semi-permeable ceramic membranes, andcirculating a carrier liquid capable of retaining TEG compounds throughbores of said hollow fibre ceramic membranes, said bores having aninside surface, wherein said exhaust gas contacts an exterior surface ofsaid membranes whereupon TEG within said exhaust gas permeates throughsaid membrane and contacts said carrier liquid below the exteriorsurface of said ceramic membranes, thereby lowering the concentration ofsaid TEG within said exhaust gas and elevating the concentration of saidTEG compounds within said carrier liquid; Discharging said exhaust gascontaining a reduced TEG concentration from the enclosed space andremoving said carrier liquid containing said TEG compounds therein fromsaid hollow fibre ceramic membrane array.
 30. The method of claim 29where said TEG within said exhaust gas permeates through said membraneand contacts said carrier liquid at the inside surface of said bores.